Elementary Geometry for College Students (7th Edition)

Using Pythagorean's Theorem... $a^{2}+b^{2}=c^{2}$ Substitute in known lengths... $11^{2}+8^{2}=15^{2}$ Simplify... $121+64=225$ $185\ne225$ Since the leg lengths (squared) do not equal the hypotenuse (squared) then the triangle is not a right triangle.